In commercially available liquid chromatography systems, it is common place to provide a plurality of solvent sources which allows the user to select which solvent or solvents will be used in a particular analytical run or experiment, and to select two or more solvents which are mixed together in desired proportions that are either fixed for a period of time or vary over a period of time, the latter operation being known as gradient programming. The trend in chromatography is towards systems providing higher and higher degrees of resolution and to accomplish this, it is necessary that when two or more solvents are used in an experiment, the solvents be mixed as thoroughly as possible. Poor or improper mixing leads to erratic and irreproducible results.
In general, the systems of the prior art allow each solvent reservoir to be connected through a selectively controllable valve whereby operation of the series of valves controls which solvent is being used and the desired proportion of plural solvents. Commonly, the valves are actuated by a solenoid which is controlled through some form of a solvent gradient or program control system. In one commercially available valve, three solenoid actuators are mounted at right angles to each other upon a valve body and control three valve members so as to allow solvents from three different reservoirs to be proportional. Mixing occurs in a chamber located downstream of the valve members and the mixing is done without any additional mechanical mixing device. In other words, the mixing occurs due to the flow conditions in the chamber. There are some disadvantages to such a system. First, the passageways are not hydraulically balanced and this causes slightly different flow characteristics and proportioning to occur dependent upon which valves are open. Second, the valve has a common outlet in line with one of the inlets but at right angles to two other inlets causing different solvents to flow under primarily laminar flow conditions in such a manner as to not promote any mixing of the solvents. Third, the valve has dead spaces located in passages downstream of the valve members which accumulate solvents when the valve members are shut. Thus, if only one valve member is open, the desired solvent then washes undesired solvent from the dead spaces of the other solvent passages.
Another problem occurs because of the occasional presence of air, within the system, which might leak in, e.g., due to poor fittings. This problem is particularly troublesome where the valve body is formed of an inert material, such as polytetrafluoroethelene. With such material, air bubbles tend to adhere to the material and it is necessary that the solvent flowing through the system wash or otherwise dislodge the air bubbles from the material and pass it through the system. However, most solvents have a low degree of wettability relative to the material and it therefore becomes difficult to dislodge the air bubbles with the result that sometimes an air blockage might occur to thereby distort proportioning.